A microprocessor, or simply processor, such as a central processing unit (“CPU”) or a graphics processing unit (“GPU”), is an integral component for computing devices, e.g., laptops and servers, and often demands high current within a short period of time, for example, 100 A in 150 nS. Likewise, a processor may also quickly transition to a low power state, for example, 100 A to 1 A in 100 nS to 150 nS. All such demands must be met by the available power supply circuit, which powers the processor. Other types of loads, such as a large application specific integrated circuit (“ASIC”), a system on a chip (“SoC”), and a field programmable field array (“FPGA”) may also exhibit similar behavior.
However, large load transients typically cause undershoot and overshoot in the supply voltage. Undershoot occurs when the transient change in the output voltage of a DC-DC converter exceeds the lower limit of the voltage specification, while overshoot occurs when the transient change exceeds the upper limit of the voltage specification. Thus, undershoot and overshoot specifications are fairly stringent for processor core power rails. A typical specification is 2 mV per ampere. As an example, for a 100 A load transient, the undershoot and overshoot specification associated with the power supply should not be more than 200 mV.
The problem of undershoot and overshoot with load transients has another dimension. Since the processor is running fast and also entering low power modes, the load transients can be in the medium frequency range of 100 Hz to 2 MHz. Typically, medium frequency load transients cause larger undershoot and overshoot than lower frequency load transients due to circulating currents. The circulating currents typically occur when the transients described above excite a resonant LC filter that is used to filter the output ripple to acceptable levels in a step-down converter. This forces the circuit designer to use higher output capacitance in order to meet the stringent undershoot and overshoot specification.
Thus, it would be desirable to handle the load transients with reduced undershoot and overshoot and without the need for increased output capacitance.